The present invention relates in general to gyroscopic sensors having a mechanical element, referred to as a xe2x80x9cresonatorxe2x80x9d, capable of vibrating on a mechanical resonance, detectors responsive to elongation imparted by the vibration to the resonator, and transducers for applying forces to the resonator, and more particularly the invention relates to gyroscopic sensors in which the resonator, in the form of a bell or a cap having a rotational symmetry is fixed along its axis and presents a circular edge situated in a plane perpendicular to the axis and at a distance from the fixing point in the axial direction.
Gyroscopic sensors are already known that use such a resonator. Embodiments can be found in U.S. Pat. No. 4,157,041 (General Motors Corporation) and U.S. Pat. No. 4,951,508. In general, the resonator is constituted by a hemispherical cap made of silica enabling very high Q-factors to be obtained, possibly in excess of 5xc3x97106. Capacitive-type detectors are placed in such a manner that the vibratory movement of the resonator in the radial direction modifies the inter-electrode distance. Transducers for exciting resonance are placed so as to exert electrostatic forces at least in two particular radial directions in which elongation is detected by variation in the inter-electrode distance.
To obtain sufficient efficiency in detection and energization, the airgaps of the detector and of the transducers must be small. In practice, the fixed electrodes of the transducers and of the detectors are placed on spherical pieces that are concave or convex depending on whether they are outside or inside the resonator, and they are adjusted to define the airgaps. In practice, it is not possible to reduce these gaps to less than 100 xcexcm to 200 xcexcm.
Such a sensor is very expensive to make. Obtaining small and identical airgaps between spherical pieces requires very high precision in manufacture and accurate concentricity. It is much more difficult to make electrodes on spherical surfaces than it is on plane surfaces. The tracks of conductors for making electrical connections and any guard electrodes around the electrodes of the detector and the transducer are likewise difficult to make in three dimensions. Finally, it is necessary for the thermal expansion coefficients of the resonator and of the spherical pieces carrying the fixed electrodes to match.
Until now, all of those constraints have limited the use of sensors having a cap-shaped or bell-shaped resonator to top-of-range applications, which can accept such high cost.
The present invention seeks to provide a gyroscopic sensor of the above-defined kind that can be made at much lower cost than presently-existing sensors.
To achieve this result, the invention makes use of an observation that has previously been overlooked, i.e. that the edge of a bell-shaped or cap-shaped resonator excited in a mode of vibration that gives rise to deformation with both radial and tangential components also presents a component of displacement parallel to the axis of the resonator. In the particular case of a hemispherical resonator, it will be shown that the amplitude of its axial displacement is equal to half the amplitude of its radial displacement.
The invention provides in particular a sensor having:
a resonator in the form of a bell or a circularly-symmetrical cap, the resonator being fixed on its axis and presenting a circular edge situated in a plane perpendicular to the axis and at a distance from the fixing point of the resonator in the axial direction; and
a substrate carrying detection and excitation means for co-operating with the resonator;
the sensor being characterized in that the detection and excitation means co-operate with an axial component of resonator vibration.
In an advantageous embodiment, said means comprise detectors and transducers which can be placed on the substrate which is orthogonal to the axis, and placed facing the edge of the resonator.
The detectors and/or transducers can have various known structures. They can have electrodes co-operating with the resonator to constitute capacitive detectors and electrodes co-operating with the resonator that constitute capacitive excitation transducers.
The detection and excitation means can be made using the same components operating in time sharing, or by modulating carriers at different frequencies. Displacements in the presence of an excitation signal can then be detected by synchronous detection. Furthermore, the detectors and/or transducers can be of kinds other than electrostatic.
The bottom of the resonator is advantageously fixed to the substrate by means made of a material having substantially the same coefficient of expansion as the resonator, so as to make the sensor very insensitive to temperature variations.
It can be seen that the sensor of the invention uses the axial component of the displacement of the edge of the resonator to detect the amplitude of the vibration at the measurement points, and also for the purpose of exciting the resonator.
When the resonator is not hemispherical, the amplitude of the axial displacement of the edge is not necessarily equal to half its radial amplitude. In particular, the amplitude of axial deformation at the edge decreases with increasing depth (axial length) of the resonator starting from the hemispherical shape.
In practice, an airgap at rest lying in the range 5 xcexcm to 100 xcexcm is adopted, and generally lying in the range 5 xcexcm to 20 xcexcm, i.e. values which are much easier to achieve with mutually confronting plane surfaces than with facing spherical surfaces. To retain acceptable dimensions, a resonator will generally be used having a natural frequency of less than 10 kHz. It is desirable for the sensor to be placed in a vacuum in order to reduce damping.
The resonator can be of constant wall thickness. It can also have greater thickness in the portion close to the edge so as to increase the effective area of the electrodes carried by the substrate. The electrodes placed on the substrate will advantageously present a radial dimension greater than that of the edge of the resonator so as to project beyond both sides thereof and so that a small amount of off-centering will have no effect on measurement.